1. Field of the Invention
The subject invention relates to an optical film assembly. Specifically, it relates to an optical film assembly of the backlight module in a liquid crystal display (LCD) apparatus.
2. Descriptions of the Related Art
Compared to other flat-panel display apparatuses, the LCD has low battery usage, is lightweight, and has high luminance. The LCD has been used with various technological developments, such as mobile phones, laptops, liquid crystal televisions, and digital cameras. Generally, the LCD apparatus comprises a section for controlling the liquid crystals and a section for providing uniform light. The section that controls the liquid crystals adjusts the light transmittance of the liquid crystals. The section that provides uniform light (i.e., the so-called backlight module) comprises a light source and an optical member for improving the characteristics of the light generated from the light source.
An LCD apparatus should have the following qualities: high luminance, high uniformity, and high contrast. To improve the brightness (luminance) of the backlight module in the LCD apparatus, the light is first directed by a light guide plate (LGP) and then is passed through additional film materials, thereby, enhancing the entire optical performance of the liquid crystal panel. The film material comprises diffusion films, brightness enhancement films (BEFs), dual brightness enhancement films (DBEFs), and polarization converters. The BEF with a prismatic structure on its surface is called a prismatic film. The DBEF is normally defined as a BEF with polarization and reflection functions.
Taiwan Patent Publication No. M289869 discloses a backlight module comprising film materials such as a diffusion plate, a prismatic film, and an upper diffusion film, in addition to a LGP, a light source, and a reflector. Depicted as the conventional backlight module 100 in FIG. 1, the prior art typically uses a first prismatic film 130 with a prismatic structure 131 on its surface, a second prismatic film 140 with a prismatic structure 141 on its surface (in which the configured directions of the prismatic structures 141 and 131 are perpendicular to each other), and an enhanced polarization converter 150 with a polarization direction 151 deposited above the LGP 110 and diffusion layer 120. The converter 150 is typically used with the lower polarizer (not depicted) of the LCD panel. Alternatively, the prior technology has also proposed a backlight module 200 depicted in FIG. 2. Particularly, a DBEF 250 with a polarization direction 251 is used instead of the enhanced polarization converter 150 shown in FIG. 1.
However, the usage of too many film materials will not only reduce light transmission, but it will also increase the thickness of the films, which influences the special use of the backlight module, resulting in an unstable elemental assembly. Because of this drawback, other integration ways have been proposed. For example, a DEBF is combined with a BEF with a prismatic structure on its surface (a prismatic film) to thin the film materials. When liquid crystals are used however, the polarization direction of the DBEF is always designed to be the same as the configured direction of the prismatic structure on the BEF. Under such circumstances, there will be high luminance from certain visual angle regions and low luminance from other visual angle regions, and thus, there will be a significant difference between luminance in difference regions, especially in the on-axis viewing locus. The on-axis viewing locus refers to the cross-locus of the X and Y axes which is swept by the observer's eyes during the panel luminance test. In the panel luminance test, the observer views the panel luminance in a direction perpendicular to the panel center (i.e., the direction parallel to the normal line of the panel) and then respectively tiles the specific angles to the right side (i.e., positive X axial direction), the left side (i.e., negative X axial direction), the upper side (i.e., positive Y axial direction), and the lower side (i.e., negative Y axial direction). The cross-locus is the region mainly viewed by users, and thus, the display manufacturers always conduct the luminance test in this region.
In particular, the entire luminance of the cross-locus viewed by users is extremely uneven. As shown in FIG. 3, each color level (gray level) in the right color bar represents the different strength in luminance where the lowest one refers to the lowest luminance and the highest one refers to the highest luminance. All the color levels (gray levels) correspond to the test results of the luminance shown in the left chart of FIG. 3. For example, in the positive X axial direction of the cross-locus, the luminance observed from position P0 perpendicular to the panel center was first measured. Then, the observer tiled his/her head with angles 15°, 30°, and 45° from the P0 to the right (i.e., the positive X axial direction) to get the positions P15, P30, and P45, respectively, and measured the luminance. The color levels (gray levels) measured at the positions P0, P15, P30, and P45 correspond to those marked by B0, B15, B30, and B45, respectively, on the right color bar. The test charts show that there is an extreme difference in luminance between the positions represented by B0 and B45 (i.e., the position perpendicular to the panel center and the position defining a 45° angle with respect thereto).
Consequently, the subject invention provides an optical film assembly which can prevent the abovementioned drawbacks of the prior art. Moreover, the optical film assembly can be used together with liquid crystals to exhibit better and more uniform optical performance at each visual angle.